Radio receiving apparatus



RADIO RECEIVING APPARATUS Filed May 12, 1932 I INVENTOR Willard plaoe BY QJZW HIS ATTORNEY Patented May 15, 1934 UNlTED STATES PATENT OFFICE The Union Switch &

Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application May 12, 1932, Serial No. 610,860

8 Claims.

The present invention relates to a new method of regulating the volume of reception in radio receivers and to a new circuit for carrying out the method which insures substantially constant volume of receiver output irrespective of the energy input to the receiver, operates practically instantaneously and permits of adjustment at a point remote from the amplifier or amplifiers controlled thereby.

The new circuit is so arranged as to insure that the amplifier or amplifiers of the receiver will be operating at maximum intensity when no signal or but a weak signal is being received and will have the amplification factor progressively l5 reduced as the energy input to the receiver increases beyond a predetermined level.

A feature of the invention is the use of an asymmetric conductor through which current is passed in one direction or the other depending upon the magnitude of the direct current component of the detector output circuit; the potential difference across the conductor being used to control the amplification preceding detection by variation of the biasing potential applied to the control grid of one or more amplifiers. By the use of such an asymmetric conductor remote manual adjustment of the volume level is made possible and a wide range of adjustment of the amplification is obtained in response to but small changes in the detector output current; amplification of the controlling current not being required. V

For a better understanding of the invention reference may be had to the accompanying drawing in which the preferred form of volume control circuit is shown in connection with a receiver utilizing a full wave metal oxide rectifier as a detector.

The receiver illustrated in the drawing comprises three stages of radio frequency amplification including the amplifiers 2, 4 and 6, a full wave rectifier 8 serving as a detector, and a stage of audio frequency amplification comprising the tube 10. Amplifier 2 is coupled to the input terminals 12 by means of a transformer 1; the primary of this transformer being connected across terminals 12 and the secondary having one end connected to the control grid of tube 2 and its other end connected to the cathode of this tube through a capacity 13. The plate circuit of tube 2 is coupled to the input circuit of tube 4 by means of a transformer 3 of which the secondary has one end connected to the grid of tube 4 and the other end connected to the cathode of this tube through a capacity 14. The purposeof capacities 13 and 14 will be made apparent hereinafter. Tubes 4 and 6 are coupled by means of transformer 5, the secondary of which has one end connected to the grid of tube 6 and its other end connected to the cathode of this tube through a suitable fixed grid biasing battery. Amplifier 6 is indicated as a large capacity tube having, in addition to a control grid and a screen grid, a cathode grid for shielding the screen grid from the secondary electrons emitted by the plate. Positive potential is applied to the anodes of tubes 2, 4 and 6 from any suitable source such as the battery 15 the entire voltage of the battery being applied to the anode of tube 6 through the primary of the output transformer '7 of this tube and a portion of the voltage of battery 15 being supplied to the anodes of tubes 2 and 4 through conductor 16 and the primaries of transformers 3 and 5 respectively. A still smaller portion of the voltage of battery 15, through conductor 17, supplies positive potential to the screen grids of tubes 2 and 4. The screen grids of tubes 6 and 10 are maintained at substantially'the potential of battery 15 by means of conductors 18 and 19 connected at one end to the positive terminal of battery 15 and at their other ends through suitable high resistances with the screen grids of these tubes.

Full wave rectifier 8 is preferably of the metal oxide type and serves as a detector of the high frequency voltage impressed upon its input terminals which are connected across the secondary of transformer '7. The output terminals of rectifier 8 are connected across the primary of an audio frequency transformer 9 to one end of the secondary of which the control grid of amplifier 10 is connected and to the other end of the secondary of which the cathode of amplifier 10 is connected through a suitable fixed grid biasing battery. Tube 10 like tube 6 is illustrated as of the large capacity type having a cathode grid in addition to control and screen grids. The anode of tube 10 is connected through the primary of an output transformer 11 with conductor 19 leading to the positive terminal of battery 15; transformer 11 serving to couple tube 10 to an output circuit including a load device indicated diagrammatically at 20. Device 20 may be for example a loud speaker, earphones or coupling to succeeding stages of amplification.

When a modulated high frequency voltage wave is impressed across the input terminals 12 of the receiver, the current through the primary of transformer 1 induces an alternating voltage in the input circuit of tube 2 which is successively amplified by tubes 2, 4 and 6 and impressed across the terminals of rectifier 8 which thereupon transmits a unidirectional current periodically varying in magnitude at the frequency and with the amplitude of the modulation current. The alternating component of this pulsating direct current is then amplified by tube 10 and transmitted by transformer 11 to the load device 20.

The above-described circuit is an example of a suitable receiving circuit with which the new volume control now to be described may be advantageously employed but it will be apparent that the invention is not limited to the combination of the new control circuit with the particular receiving circuit illustrated as the new control circuit could be used in combination with other types of receivers to obtain some or all of the advantages inherent to the particular disclosure of the present application.

In order to control the amplification inresponse to the magnitude of the direct current component rent corresponding to the modulation current.-

The high potential end of resistor 22 is grounded and the low potential end thereof is connected through a resistor 24 and asymmetric conductor 26 with a tap 2'7 along a potentiometer resistance 28 across which a steady direct voltage is applied by a battery 29; the positive terminal of battery 29 and one end of resistor 28 being grounded.

Conductor 26 may be, for example, one element of a copper oxide rectifier so connected between tap 2'7 and resistor 24 as to offer but small resistance to the fiow of current in the direction from resistor 24 to tap 2'7 but to offer a relatively large resistance to fiow of current in the reverse direction. With this arrangement if the values of resistance 28 and battery 29 are so chosen and the position of tap 27 so selected that the negative potential at tap 27 substantially equals the average output voltage of the rectifier when the volume of reception is that corresponding to full loading of amplifier 10, then there will be no current through element 26 when the rectifier output is maintained at the desired level but a current will flow through this element in one direction or 1e other when the volume level departs from the of resistor 22 being lower than that at tap 2'? 'current fiows through element 26 in the high resistance direction. When the rectifier is delivering a voltage less than that at which the volume level is to be maintained the potential atthe ungrounded end of resistor 22 will be less negative than that at tap 27 and current will flow in the low resistance direction through element 26. Under these circumstances, however, as the resistance of element 26 is small in this direction the potential at point 25 will not exceed that at tap 27 by any substantial amount. For example, if the average voltage across resistor 22 is about 30 volts when the amplifier 10 is fully loaded and tap 27 is accordingly positioned to be at about 30 volts negative with respect to ground, then resistances 22 and 24 may be so selected as to insure that the maximum potential at point 25, corresponding to no output voltage of the rectifier S, will be about 28 volts negative with respect to ground or 2 volts positive with respect to the potential of tap 27.

As amplifiers such as the tubes 2 and 4 can have their amplification factor varied over a wide range by but a small change in the negative potential of their control grids it will be apparent that element 26, the potential across which responds rapidly to variations in the direct current output of the rectifier provides a ready means for controlling the amplification factor of these tubes. For this purpose it is but necessary to connect point 25, with the input circuits of tubes 2 and 4 respectively and to apply to the cathodes of these tubes a constant potential, as for example by connecting the cathodes of these tubes with a tap 30 on resistor 28.

Assuming, as before, that the negative potential at tap 27 is about 30 volts, then if that at tap 30 is about 26 volts, there will be a negative biasing potential of about 2 volts, applied to the control grids of tubes 2 and 4 when no voltage is impressed upon the receiver terminals 12; the two volts corresponding to a potential drop of four volts between taps 30 and 27 and a rise of two volts across element 26. The amplifiers will thus be operating at substantially maximum sensitivity. As the rectifier output increases from zero to 30 volts the sensitivity of the amplifiers will still be high as this biasing potential will vary only slightly throughout this range of rectifier output voltage; the negative biasing potential increasing very slowly to 4 volts when the rectifier output reaches 30 volts and no current fiows through element 26. With further increase in rectifier output however, the sensitivity of the amplifiers will be rapidly decreased as the biasing potential applied to the grids of tubes 2 and 4 increases rapidly when the current through element 26 fiows in' the direction in which the element offers high resistance. With certain tubes a variation of about ten volts in the negative biasing potential of the control grid varies the amplification factor from maximum sensitivity where the amplification is greatest to minimum where the tube operates as an attenuator. This range of control is readily obtainable by means of the control circuit above described as the potential at point 25 can be made to Vary through more than a 10 volt range in response to the output current of the rectifier.

In order to filter out the lower audio frequencies from the control circuit isolating resistors 31 and 32 are inserted in the leads 33 and 34 connecting point 25 with the input circuits of tubes 2 and 4 respectively and these resistors together with the by-pass capacities 13 and 14 comprise the only filtering necessary. Since the time lag due to filtering ofthis circuit will be practically negligible and since the speed of operation of the automatic volume control is governed by the time lag of the filter, it is apparent that the volume control of the invention can be made to operate in a small fraction of a second.

The level at which the output voltage of the rectifier is maintained by the new volume control circuit may be adjusted by moving taps 27 and 30 simultaneously along the potential divider 28 so that the potential between these taps remains trolled amplifiers. Since there is nothing but direct current carried by the potential divider, it

may be set up at some point remote from the receiver for remote adjustment of the Volume control.

The invention has now been described in connection with a receiver utilizing a particular type of amplifier. It will be understood, however, that the new method may be employed and the new circuit used with any type of receiver in which a modulated carrier frequency is amplified and then rectified or detected. The receiver should employ amplifiers in the stage or stages to be controlled which require a controlling voltage small in comparison with the voltage delivered by the rectifier or detector in order to secure maximum advantage from the control circuit. If tubes refi substantially instantaneous control is effected quiring a higher range of controlling voltage are desirable for other reasons, then the control voltages applied to the grids of these tubes should be correspondingly amplified as by means of any suitable direct current amplifier such as are now well known. Although a receiver utilizing a full wave metal oxide rectifier as a detector is preferred because of the relatively high output voltage of such a rectifier as compared with that obtainable from a tube detector various benefits would be obtained from the use of the new circuit in conjunction with receivers utilizing other types of detectors.

Thus the invention in its broadest aspect is not concerned with the particular elements of the receiver whose volume is to be controlled but is directed to a volume control utilizing an asymmetric conductor through which the direction of cur-- rent fiow is controlled in response to the magnitude of the detector output voltage and which is so arranged that the potential difference thereacross reacts upon the amplifiers to vary their amplification factors in a direction to maintain constant the output voltage of the detector at a desired volume level; the control being such that and adjustment of the volume level remote from the receiver is practicable.

The following is claimed;

1. A method of using an asymmetric conductor 'in controlling the volume of reception in radio receivers which comprises varying the direction of current through the conductor in response to the magnitude of the direct current component of a rectified modulated carrier wave and utilizing resulting variations in the potential difference across the conductor to control the amplification preceding rectification in a direction tending to maintain constant rectifier output current.

2. A method of using an asymmetric conductor in controlling the volume of reception in radio receivers which comprises rectifying both halves of an amplified modulated carrier wave, amplifying the alternating component of the rectified wave corresponding to the modulation frequency,

itransmitting direct current in the low resistance direction through the conductor when the voltage of the rectified wave is less than a predetermined value, transmitting direct current through the conductor in the high resistance direction when 1 the voltage of the rectified wave is greater than the predetermined value and controlling the amplification preceding rectification in response to the potential difference across the asymmetric conductor.

amplify and detect modulated carrierfrequency voltage waves of a volume-control circuit including an asymmetric conductor, means for applying a potential difference across said conductor in one direction when the magnitude of the detector output voltage exceeds a predetermined value and in the other direction when the magnitude of the detector output voltage decreases below said value, and means for controlling the amplification preceding rectification in response to the potential difference across said conductor.

4. In combination, a receiver having a stage of high frequency amplification, a detector coupled thereto and a low frequency stage of amplification coupled to the output circuit of the detector,

an asymmetric conductor, means for applying a substantially constant negative potential to one side of said conductor, a connection including a resistance between the other side of said conductor and the low potential side of the detector output circuit, the high potential side of the detector output circuit being grounded, a resistor in the detector output circuit connected between the high and low potential sides thereof, and means for controlling the amplification factor of said high frequency stage of amplification in response to the potential difference across said conductor, said conductor being so connected with said resistances as to offer higher resistance to current flowing toward the detector output circuit through said conductor than to current flowing in the opposite direction.

5. A volume control circuit for radio receivers of the type including at least one radio frequency amplifier having a control grid, a full wave metal oxide rectifier serving as a detector and an output circuit associated with the rectifier, said volume control circuit comprising in combination a potential divider, and a source of direct electromotive force therefor, an asymmetric conductor comprising one element of a metal oxide rectifier having one side connected to a tap along said divider and the other side connected to the low potential output terminal of said full wave rectifier, the high potential end of said divider and high potential terminal of said full wave rectifier both being grounded and said asymmetric conductor being so connected as to offer relatively high resistance to current flowing from said divider to the rectifier output circuit, a resistor in the connection between said conductor and the low potential output terminal of the rectifier and a connection from the side of said conductor connected to said resistor to the input circuit of the amplifier whereby the amplification factor of the radio frequency amplifier may be controlled in response to the direction and magnitude of the current through said asymmetric conductor.

6. A volume control circuit according to claim 5 wherein the connection from the side of said conductor adjacent said resistor is connected to the control grid of the radio frequency amplifier and wherein a second tap along said divider at a potential higher than that applied to the other side of said conductor by said first mentioned tap is connected to the cathode of the radio frequency amplifier whereby the biasing potential applied to the grid of the amplifier is the algebraic sum of the potential difference between said taps and the potential difference across said conductor.

7. The combination with a radio receiver having at least one high frequency amplifier, a detector coupled thereto and an audio frequency amplifier coupled to the output'circuit of the detector, of a volume control circuit including a potential divider, an asymmetric conductor having one side connected to the control grid of said amplifier and to the low potential side of the detector output circuit and having its other side connected to said divider, an electrical connection between the divider and the cathode of said amplifier to maintain the same at a higher potential than that at the point where said condoctor is connected to said divider, a resistor in the connection between said conductor and the low potential side of the detector output circuit, and means for maintaining the high potential side of the detector output circuit and of the divider at the same potential whereby the direction of current through said conductor and resistor depends upon the magnitude of the output voltage of the detector, said asymmetric conductor offering greater resistance to the flow of current from said divider to the detector output circuit than to flow of current in the reverse direction.

8. The combination according to claim '7 including a resistor in the connection to the control grid of the amplifier and a capacity connected between the control grid and cathode of the amplifier for filtering low frequency currents from said control circuit.

WILLARD P. PLACE. 

